1. Field of the Invention
The present invention relates to a process for preparing biaxially oriented shaped articles formed from thermoplastic materials and, more particularly, to annealing, and thereafter heat setting in a second mold polyethylene terephthalate bottles which can be subjected to washing and reuse.
2. The Prior Art
Refillable plastic bottles reduce landfill and recycling problems of disposable plastic beverage bottles and, more particularly, those bottles formed from polyethylene terephthalate or PET.
A refillable plastic bottle must remain aesthetically pleasing and functional over numerous washings and refillings as discussed by U.S. Pat. Nos. 4,755,404; 4,725,464 and 5,066,528. Cracks, color changes, volume or structural change must be minimized.
U.S. Pat. No. 4,385,089, teaches how hollow, biaxially oriented shaped articles are formed from intermediate products which may be sheets or other shapes when thermoformed or parisons or preforms when injection molded, injection blown or extrusion blown. The preform may be prepared and immediately used hot or may be stored and reheated later to a temperature having sufficient elasticity to be shaped into a bottle or other form by drawing and blowing in a cooled mold to obtain the final shape of the article. The preform is next often subjected to a heat setting at well above the glass transition temperature of the thermoplastic to increase the articles strength and resistance to gas loss. Heat setting also prevents distortion when the bottle is reused, including a hot caustic wash.
U.S. Pat. No. 4,233,022, teaches the use of a first cooled blow mold for shaping a bottle and for obtaining biaxial orientation and the transfer of the bottle to a second mold shaped cavity having segmented portion separated by insulating sections to heat set the bottle to 150.degree. C. to 220.degree. C.
U.S. Pat. No. 5,085,822, teaches it is old to blow in a mold at 130.degree. C. and cool it to 100.degree. C. to prevent deformation on removal. A container may be retained in the blow mold and heated to remove stress and thereafter be transferred to a separate cooled mold to solidify. A molded container can be held for a predetermined period of time to heat set followed by the introduction of a cooling fluid into the bottle. Also disclosed is heat setting a blown container in a separate mold.
U.S. Pat. No. 4,505,664, teaches transporting the blowing cavity and blown article to a second station where a medium is circulated through the article.
U.S. Pat. No. 4,988,279, biaxially orients the article which can then be heat set.
U.S. Pat. No. 5,080,855, teaches blow molded articles which may be heat set in a second mold. Also see also U.S. Pat. Nos. 4,485,134, 4,871,507 and 4,463,121 that discuss heat treating biaxially oriented bottles.
U.S. Pat. No. 4,572,811, teaches heat treating non-oriented PET which forms an opaque wall which it has been found leads to stress cracking when bottles are recycled.
U.S. Pat. No. 4,588,620, teaches preforms having a thinner bottom wall which permits longer or deeper stretch of the shoulder and sidewall portions,
While it is known to biaxial stretch a preform using pressure, we have found those annealing of the blown preform and thereafter heat treating the container in a separate mold improves the number of times one can reuse the container.